I knew Omega was gonna be DLC, if you go to the galaxy map and put the marker to where the omega nebula is, it will play the noise that goes off when you drag the cursor over a system.

I knew (or at least expected) that it was going to be DLC as soon as I heard Aria say that she was going to retake the station from Cerberus.
After seeing that it wasn't an actual mission in the game, I knew that we could expect it to be DLC.